1. Field of the Invention
This invention relates to a method and apparatus for locating a seismic detector in a borehole so that high-resolution reverse vertical seismic profile (VSP) measurements can be obtained. The detector is a high resolution three-component sensor using accelerometers that operate in the frequency range of 100 to 1000 hertz. The accelerometers are embedded in a meltable substance such as wax. Heaters located in the meltable substance melt the wax. Upon solidification of the wax, the wax secures the seismic detector in a borehole. Thereafter, the seismic detector can be removed by the heaters again melting the wax and pulling on a line attached to the seismic detector to remove the seismic detector from the borehole.
2. Description of the Prior Art
High resolution reverse VSP (vertical seismic profile) measurements require that three-component vertical seismic signals be detected at or near the surface. Reverse VSP measurements are accomplished by having a source of seismic waves in the geological formations of interest. For example, a source of seismic signals may be located in a deep borehole (such as an oil well) with the seismic waves being radiated through the geological formations to seismic detectors deployed either on the surface or in shallow boreholes surrounding the deep borehole that contains the source of the seismic waves.
A source of seismic waves could include other apparatuses as invented by co-inventor Thomas Edwin Owen including (A) U.S. Pat. No. 4,651,311 entitled "Electrodeless Arc Discharge Acoustical Pulse Transducer" by Owen and Shroeder, (B) U.S. Pat. No. 4,706,227 entitled "Asymmetrical Lateral-Force Seismic Source Transducer" by Owen and Shroeder, (C) U.S. Pat. No. 4,525,645 entitled "Cylindrical Bender-Type Vibration Transducer" by Shirley and Owen, (D) pending U.S. patent application No. 07/698,800 by Owen entitled "Variable Multi-Stage Arc Discharge Acoustical Pulse Source Transducer."
The patents and applications indicated hereinabove are simply illustrative of different types of devices that can be used to generate a source of seismic waves. Other types of devices can be used, but the devices should be capable of generating frequencies in the range of 100 to 1000 hertz.
Conventional devices that generate seismic waves typically utilize small explosive devices in the source borehole to produce the seismic source signals (seismic waves). Vertical component geophones can be coupled by spikes into the ground surface around the source borehole. Utilizing this conventional method, the frequency range of the seismic signals generated and detected is approximately 10 to 100 hertz.
High resolution reverse VSP measurements employ a borehole seismic source device that is capable of generating seismic signals in the frequency range of approximately 100 to 1000 hertz or higher, which signal must be detected at the ground surface around the source borehole. The seismic source transducer may either be a pneumatically or hydraulically operated vibrator or mechanical impulse generator, an electric arc discharge pulse generator, or a piezoelectric device capable of generating controlled seismic wave forms radiated into the surrounding geological formations.
The seismic detector preferred for such high resolution reverse VSP measurements are acceleration sensors capable of responding accurately to the higher frequency seismic signals and capable of separately sensing the three dimensional (XYZ-axis) wave motions. These high resolution detectors are more expensive than the conventional geophones.
Effective high frequency seismic wave detection requires the sensors to be rigidly coupled to the formation in which the seismic waves propagate in order to prevent spurious responses and/or mechanical resonances caused by imperfect coupling between the sensor package and the borehole. The seismic detector package containing the acceleration sensors must be rigid and lightweight. Also, the average density of the entire detector package should be approximately the same as the average density of the geological formation to which the detector package is coupled.
The coupling method used in present-day borehole seismic detectors varies. In one embodiment, mechanical locking arms are actuated outward from the sensor probe housing to contact the borehole wall and forcibly lock the probe in place for the temporary time period required for the measurements. In general, seismic detectors that use wall-lock probes of this type have an average density which is much greater than the geological formations in which they are coupled. As a result, a very high mechanical coupling force is required to lock the sensor probes to the borehole wall. This technique is usually only partially successful in achieving the desired degree of coupling because of spurious mechanical resonances that occur in the clamping mechanism.
Alternatively, permanent installation of seismic detectors is occasionally desirable. A common practice used in such permanent installations is to embed the sensor in the borehole using Portland cement or other rigid casting material. In this case, the seismic detector package must be considered expendable because it cannot be recovered in a cost efficient manner. Permanent emplacements have been used in a number of field tests using relatively inexpensive geophones, which were abandoned when the tests were completed. This method of coupling is effective in providing good seismic couplings because the cement forms a rigid conformal casting in the borehole. Also, the overall density of the seismic detector and cement is reasonably well matched to that of the surrounding geological materials.
However, the acceleration sensors mentioned hereinabove for reverse VSP measurements are significantly more expensive than geophones and, therefore, cannot be considered to be expendable. The present invention pertains to a novel method of coupling seismic wave sensors in boreholes so as to achieve (A) the desirable results of rigid, conformal borehole coupling, (B) an approximate match between the average density of the sensor package and the geological formation and (C) the ability to recover the sensor package instead of abandoning it after the seismic measurements are complete. This was accomplished by having accelerometers embedded with heating coils in a meltable substance such as wax. By heating the wax through the heating coils, the wax will melt and conform to the borehole size when solidified. After use, the wax is again melted to allow removal of the entire sensor package.
In an effort to determine the closest prior art, a patentability search was performed and the following United States patents were discovered:
1. Meadows, "Method of Locating a Member in a Borehole," U.S. Pat. No. 4,879,695 issued on Nov. 7, 1989. PA0 2. Brownlow, "Seismometer Case Cover," U.S. Pat. No. 2,477,172 issued on July 26, 1949. PA0 3. O'Brown, "Apparatus and Method for Detecting Seismic Waves," U.S. Pat. No. 4,534,020 issued on Aug. 6, 1985.
However, the closest reference was Russian Patent No. 1,260,897 Al entitled "Device for Sealing Mouths of Uphill Boreholes During Logging," which shows the use of paraffin to seal a borehole that is filled with water. The objective in the Russian patent is to retain water for upwardly extending logging probes that require water as part of their coupling in the drilled formation. The paraffin wax is simply a plug feature at the entry collar of the borehole. Electrical heating elements may be used to soften the paraffin so the probe may be moved and relocated without disturbing the water tight plug. The Russian patent does not suggest the use of paraffin as a rigid coupling for the geophysical probe and, more generally, the water sealing function of an upwardly sloped borehole is not relevant to the subject invention.